Mlu-based magnetic device having an authentication and physical unclonable function and authentication method using said mlu device

ABSTRACT

A MLU-based magnetic device including a plurality of MLU-based magnetic cells, each MLU cell including a first ferromagnetic layer having a first magnetization, a second ferromagnetic layer having a second magnetization, and a spacing layer between the first and second ferromagnetic layers. An input device is configured for generating an input signal adapted for changing the orientation of the first magnetization relative to the second magnetization and vary a resistance of the MLU device. A bit line is configured for passing a sense signal adapted for measuring the resistance. A processing unit is configured for computing an electrical variation from the sense signal and outputting an electrical variation signature. The present disclosure further pertains to an authentication method for reading the MLU device.

FIELD

The present invention concerns a magnetic logic unit (MLU)-basedmagnetic device configured for performing an authentication function andfor generating physical unclonable functions. The present inventionfurther concerns an authentication method using the MLU-based magneticdevice.

DESCRIPTION OF RELATED ART

Development of Physical Unclonable Functions (PUFs) is currently a hottopic in the field of security.

PUFs can extract chip-unique signatures from integrated circuits (ICs)by exploiting uncontrollable randomness due to manufacturing processvariations. These signatures can then be used for many hardware securityapplications including authentication, anti-counterfeiting, IC metering,signature generation, and obfuscation. These PUF rely on physicalproperties to allows unambiguous, intrinsic and unforgeableidentification of a device.

PUFs are based on a challenge/response principle, where theauthentication is performed by providing an input signal (the challenge)to the PUFs, and comparing its output (the response) to a referencecorresponding to the identity to match.

One of the main property of PUFs is that the device is one-way, whichimplies that the response cannot be retraced unambiguously to itschallenge, but a single challenge always results in an identicalresponse.

A typical example of authentication is for software licensing, where alicense is activated for only a single device, where a PUF wouldcompletely prevent hacking and impersonation.

However, as of now, very few devices corresponds strictly to all thecriterion for a PUF. The only referenced example of a true PUFfunctionality involves optics, with a laser (challenge) being passedthrough a transparent medium where imperfections induces a uniquescattering pattern (response). But this solution is cumbersome anddifficult to miniaturize.

US2015071432 discloses to PUFs-based on an array of magnetoresistiverandom-access memory (MRAM) cells. A challenge to the array of MRAMcells may identify some of the cells to be used for the physicallyunclonable function. A response to the challenge may be obtained foreach cell by using the resistance(s) of one or both of the MTJs for acell to obtain a value that serves as the response for that cell.

US2015071431 describes a method of implementing a PUF. The methodincludes exposing an array of MRAM cells to an orthogonal externalmagnetic field. The MRAM cells are each configured to represent one of afirst logical state and a second logical state, and the orthogonalexternal magnetic field is oriented in an orthogonal direction to aneasy axis of a free layer of the MRAM cells to place the MRAM cells in aneutral logical state that is not the first logical state or the secondlogical state. The method further includes removing the orthogonalexternal magnetic field to place each of the MRAM cells of the arrayrandomly in either the first logical state or the second logical state.

US2017170827 discloses magnetic tunnel junction (MTJ) structures andpermanent bit strings used for chip identification and/orauthentication. The MTJ element structures can have a defined resistanceprofile such that the intrinsic variability of the MTJ element structureis used to encode and generate a bit string that becomes a fingerprintfor the chip.

US2013073598 concerns an entropy source and a random number (RN)generator. In one aspect, a low-energy entropy source includes amagneto-resistive (MR) element and a sensing circuit. The MR element isapplied a static current and has a variable resistance determined basedon magnetization of the MR element. The sensing circuit senses theresistance of the MR element and provides random values based on thesensed resistance of the MR element.

SUMMARY

The present disclosure concerns a MLU-based magnetic device comprisingone or a plurality of MLU-based magnetic cells, each MLU cell comprisinga first ferromagnetic layer having a first magnetization, a secondferromagnetic layer having a second magnetization, and a spacing layerbetween the first and second ferromagnetic layers; an input deviceconfigured for generating an input signal (challenge) adapted forchanging the orientation of the first magnetization relative to thesecond magnetization and vary a resistance of the MLU device; a bit lineconfigured for passing a sense signal adapted for measuring saidresistance; and a processing unit configured for computing an electricalvariation from the sense signal and outputting an electrical variationsignature.

In an embodiment, the processing unit can comprise a comparison circuitconfigured for comparing the electrical variation signature to areference electrical variation signature computed for a reference MLUdevice and outputting an output signal comprising information on thematch or non-match between the electrical variation signature and thereference electrical variation signature.

The present disclosure further concerns an authentication method usingthe MLU device, comprising:

providing an input signal in order to change the orientation of thefirst magnetization relative to the second magnetization and vary aresistance of the MLU device;

providing a sense signal in the bit line for measuring said resistance;and

computing an electrical variation from the sense signal and outputtingan electrical variation response (or signature).

In an embodiment, the input signal comprises a read magnetic fieldprovided by a field device. The field device can comprise a field lineadapted for passing a read current that generates the read magneticfield.

In another embodiment, the electrical variation comprises a noisespectral density.

Due to process variability, each MLU cell has individual characteristicsrelated to the magnetic portions of the MLU cell layers, such astransition fields, domain wall movement or unpinning of magnetization,or any other magnetic behavior inside the magnetic portions that induceselectrical variations. Furthermore, each MLU cell has individualelectrical variation characteristics. These characteristics can becombined in the electrical variation signature of the MLU device.

Due to the randomness of process induced variabilities and to the one orplurality of MLU cells comprised in the MLU device, the electricalvariation signature of an MLU device is both unique and unreproducible,like a fingerprint, ensuring the impossibility of manufacturing acounterfeit.

The MLU device can thus be configured for performing an authenticationfunction and for generating physical unclonable functions. Challengingthe MLU device can be done by measuring the electrical variation of theMLU device at a single or several set frequencies or a bandwidth, for aplurality of magnetic fields applied on the MLU cells.

The plurality of magnetic fields can be applied by means of a pluralityof currents passed through metallic lines addressing the MLU cells or byother field device capable of generating suitable said plurality ofmagnetic fields.

Such electrical variation response may then be compared, by usingcharacteristics features like peaks and valleys of the computedelectrical variation, to a reference electrical variation correspondingto the device to authenticate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof an embodiment given by way of example and illustrated by the figures,in which:

FIG. 1 illustrates a MLU-based magnetic cell, according to anembodiment;

FIG. 2 shows a cross section view of an MLU device comprising aplurality of MLU cells, according to an embodiment;

FIG. 3 shows a top view of an MLU device comprising a plurality of MLUcells, according to another embodiment;

FIG. 4 compares the measured noise spectral density as a function of aread magnetic field, for three different MLU devices;

FIGS. 5 and 6 illustrate schematically an authentication method usingthe MLU device, for a matching noise spectral density (FIG. 5) and anon-matching noise spectral density (FIG. 6).

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

FIG. 1 illustrates a MLU-based magnetic cell 10, according to anembodiment. The MLU cell 10 includes a first ferromagnetic layer 21, asecond ferromagnetic layer 23, and a spacer layer 22 that is disposedbetween the first ferromagnetic layer 21 and the second ferromagneticlayer 23.

Each of the first and second ferromagnetic layers 21, 23 can include, orbe formed of, a magnetic material and, in particular, a magneticmaterial of the ferromagnetic type. The first ferromagnetic layer 21, orsense layer, can include a soft ferromagnetic material, namely onehaving a relatively low coercivity, while the second ferromagnetic layer23 can include a hard ferromagnetic material, namely one having arelatively high coercivity. In such manner, a first magnetization, orsense magnetization, 210 of the first ferromagnetic layer can be used asa sense layer, i.e., can be readily varied under low-intensity magneticfields at which a second magnetization, or reference, 230 of the secondferromagnetic, or reference, layer 23 remains fixed and is used as areference layer. Suitable ferromagnetic materials include transitionmetals, rare earth elements, and their alloys, either with or withoutmain group elements. For example, suitable ferromagnetic materialsinclude iron (“Fe”), cobalt (“Co”), nickel (“Ni”), and their alloys,such as permalloy (or Ni80Fe20); alloys based on Ni, Fe, and boron(“B”); Co90Fe10; and alloys based on Co, Fe, and B. In some instances,alloys based on Ni and Fe (and optionally B) can have a smallercoercivity than alloys based on Co and Fe (and optionally B).

Other implementations of the sense layer 21 and the reference layer 23are contemplated. For example, either, or both, of the sense layer 21and the reference layer 23 can include multiple sub-layers in a fashionsimilar to that of the so-called synthetic antiferromagnetic layer.

Other implementations of the MLU cell 10 are contemplated. For example,the relative positioning of the first ferromagnetic layer 21 and thesecond ferromagnetic layer 23 can be reversed, with the sense layer 21disposed above the second ferromagnetic layer 23.

The spacer layer 22 can be a non-magnetic conductive layer, such as in agiant magnetoresistance junction or an insulator, such as in a tunnelmagnetoresistance junction.

In the latter case, the tunnel barrier layer 22 can include, or beformed of, an insulating material. Suitable insulating materials includeoxides, such as aluminum oxide (e.g., Al₂O₃) and magnesium oxide (e.g.,MgO). A thickness of the tunnel barrier layer 22 can be in the nm range,such as from about 1 nm to about 10 nm.

The MLU cell 10 further comprise a bit line 3 electrically contactingone end of the MLU cell 10 and the other end of the MLU cell 1 such thata sense current, or sense signal, 31 can be passed in the MLU cell 10.The MLU cell 10 further includes an input signal 44 (challenge)configured for changing the orientation of the sense magnetization 210relative to the reference magnetization 230 and vary a resistance of theMLU device.

In an embodiment, the input signal can comprise a read magnetic field 44provided by a field device. In the particular embodiment illustrated inFIGS. 1 and 2, the field device includes a field line 4 arranged forpassing a read current 43 generating the read magnetic field 44.

A MLU-based magnetic device 100 can comprise a plurality of MLU cells 10electrically connected in any series or parallel arrangement.

FIG. 2 shows a cross section view of an exemplary MLU device 100comprising a row containing a plurality of MLU cells 10, each MLU cells10 being connected in series via the bit line 3. The MLU device 100 cancomprise a plurality of rows arranged in the plane x-z (extendingperpendicular to the page) such as to form an array of row and columns.The field line 4 addresses each MLU cells 10. The field line 4 can becontinuous, for example by having the shape of a serpentine runningthrough the array, in the plane x-z, or the field line 4 can comprise aplurality of field line portions, extending perpendicular to the row ofMLU cells 10.

The MLU device 100 can be written (or programmed) by providing a datastate to each of the MLU cells 10. This can be performed by passing awrite current 41 in the field line 4. The write current 41 can beconfigured such as to generate a write magnetic field 42 adapted forswitching the direction of the reference magnetization 230 in each ofthe MLU cells 10.

The specific combination of the written direction of the referencemagnetization 230 in each of the MLU cells 10 corresponds to aprogrammed state of the MLU device 100.

Depending on the field line 4 configuration, the write magnetic field 42switches the reference magnetization 230 of all the MLU cells 10 in thesame direction or in different directions. This can be achieved, forexample, when a single field line 4 is used such that a single writemagnetic field 42 is generated in one direction. Alternatively, and asshown in FIG. 2, the field line 4 can be configured to generate aplurality of write magnetic fields 42 such that the referencemagnetization 230 of a MLU cell 10 is aligned in a direction opposed tothe one of the reference magnetization 230 of the adjacent MLU cell 10.

In a variant, the MLU cell 10 comprises an antiferromagnetic layer 24disposed adjacent to the second ferromagnetic layer 23 and, throughexchange bias, pins the reference magnetization 230 along a particulardirection when a temperature within, or in the vicinity of, theantiferromagnetic layer 24 is at a low threshold temperature T_(L),i.e., below a blocking temperature, such as a Neel temperature, oranother threshold temperature of the antiferromagnetic layer 24. Theantiferromagnetic layer 24 unpins, or frees, the reference magnetization230 when the temperature is at the high threshold temperature T_(H),i.e., above the blocking temperature, thereby allowing the referencemagnetization 230 to be switched to another direction. In thisconfiguration, writing the MLU device 100 can comprise a step of heatingthe MRAM cells 10 at the high threshold temperature T_(H) such as tounpin the reference magnetization 230 when the programming magneticfield 42 is generated. This can be performed by passing a heatingcurrent 32 in the bit line 3. Once the reference magnetization 230 hasbeen switching in the written direction, the MRAM cells 10 can be cooleddown to the low threshold temperature T_(L), for example by stopping theheating current 32 from passing in the bit line 3, such as to pin thereference magnetization 230 in the written direction.

Alternatively, the step of heating the MRAM cells 10 at the highthreshold temperature T_(H) can be achieved by passing the heatingcurrent 32 in the field line 4. In this alternative, it is assumed thatthe heat generated by Joule effect through the field line 4 when passingthe heating current 32 is transmitted to the MLU cells 10 by thermalconduction through an electrically non-conductive, dielectric/oxidelayer (not shown) between the field line 4 and the MLU cells 10.

Other configurations of the field line 4 can be envisaged such as toswitch the reference magnetization 230 of the MLU cells 10 according toany other patterns.

Other arrangements of the MLU device 100 can also be contemplated, forexample wherein the plurality of MLU cells 10 are electrically connectedin parallel.

FIG. 3 shows a top view of another exemplary MLU device 100 wherein theMRAM cells 10 are arranged according to a Wheatstone bridge, or fullbridge, circuit configuration. In particular, the MLU device 100comprises a bit line 3, forming a first branch 3 a and connecting inseries at least two MLU cells 10. The MLU device 100 further comprisesanother bit line 3, forming a second branch 3 b and connecting in seriesat least two MLU cells 10, the two branches 3 a, 3 b being electricallyconnected in parallel. The field line 4 can address the MLU cells 10 bybeing configured for passing the write current 41 and the input signal43. More generally, the plurality of MRAM cells 10 can be arranged suchas to form a half-bridge or full-bridge circuit configuration. Theplurality of MRAM cells 10 can also be arranged in single branch.

According to an embodiment, an authentication method using the MLUdevice 100 can comprise the step of providing the input signal 43 inorder to change the orientation of the sense magnetization 210 relativeto the reference magnetization 230 and vary a resistance of the MLUdevice 100.

The step of varying the orientation of the sense magnetization 210 cancomprise passing the input signal 43 in the field line 4 in order togenerate a read magnetic field 44 addressing the plurality of MLU cells10.

The magnitude of the input signal 43 can be varied such as to vary theintensity of the read magnetic field 44 applied on the MLU cells 10. Thevarying read magnetic field 44 changes the orientation of the sensemagnetization 210 in the plurality of MLU cells 10 but not that of thereference magnetization 230. The input signal 43 thus modulates theresistance of each of the MLU cells 10.

The magnitude of the input signal 43 can be varied such that the readmagnetic field 44 is slowly stepped, for example in 1-2 Oe increments.At each increments step, the MLU device 100 may be allowed toequilibrate (for example for 2 minutes).

The authentication method further comprise a step of providing the sensesignal 31 in the bit line 3 for measuring a total resistance R of theMLU device 100.

The authentication method further comprise a step of computing anelectrical variation from the sense signal 31 and outputting anelectrical variation response (or signature). The electrical variationresponse obtained from the sense signal 31 comprises a signaturecharacteristic of the sense magnetization 210 switching properties foreach of the MLU cells 10.

Depending on the field line 4 configuration, the read magnetic field 44addressing the plurality of MLU cells 10 can differ between the MLUcells 10. For example, the field line 4 can be arranged such that theread current 43 passes in a first direction the field line 4 addressinga subset of a plurality of MLU cells 10 and in a second direction,opposed to the first direction, in the field line 4 addressing the otherMLU cells 10. The read magnetic field 44 will thus be applied on thesubset of MLU cells 10 in a direction opposed to the one applied on theother MLU cells 10. Such a field line 4 configuration is illustrated inFIG. 2.

Other possible field line 4 arrangements can include a field line 4configured for generating a read magnetic field 44 applied to the MLUcells 10 with a single direction, or a plurality of read magnetic fields44, each having a different direction and being applied to a differentsubset of MLU cells 10.

The different field line 4 arrangements above will result in a differenttotal resistance R of the MLU device 100 and thus, in a differentelectrical variation response.

It should be understood that the present invention is not limited to theexemplary embodiments described above and other examples ofimplementations are also possible within the scope of the patent claims.

For example, the field device can be configured to generate the writemagnetic field 42 and/or the read magnetic field 44 (or input signal)without the MLU device 100 requiring the field line 4. In a possibleembodiment, the field device generate magnetic field by means of anelectromagnet. In another possible embodiment, the field device generatemagnetic field by means of permanent magnets. In this configuration, thefield device can be configured for generating the write magnetic field42 such that the direction of the reference magnetization 230 in each ofthe MLU cells 10 can be switched. The field device can be furtherconfigured for providing the read magnetic field 44 addressing theplurality of MLU cells 10.

In yet another embodiment, the input signal can include a supply voltageor current, a temperature variation or any other input signal that isadapted for switching the sense magnetization 210.

During the reading step of the MLU device 100, the field device can beconfigured such as to provide a varying read magnetic field 44 thatchanges the orientation of the sense magnetization 210 in the pluralityof MLU cells 10 but not that of the reference magnetization 230. Thefield device can be further configured such that the read magnetic field44 can be slowly stepped, for example in such a way as already describedabove.

In another possible embodiment, when writing the MLU device 100, thedirection of the reference magnetization 230 in each of the MLU cells 10can be switched by using an annealing treatment. For example, thereference magnetization 230 aligns parallel to the anisotropy axis ofthe reference layer during annealing.

The total resistance R of the MLU device 100 can be measured in a singleoperation. Such sensing operation is described in more details in patentapplication US2013241536 by the present applicant. The sense signal 31,or a voltage corresponding to the sense signal 31, corresponds to asignal that comprises a signature characteristic of the sensemagnetization 210 switching properties for each of the MLU cells 10.

Due to process variability during the manufacturing of the MLU device100, each MLU cell 10 can have characteristics related to the magneticportions of the MLU cell layers, such as transition fields, domain wallmovement or unpinning of magnetization, or any other magnetic behaviorinside the magnetic portions that induces electrical variation that isdistinct from the one another MLU cell 10. Here, the transition field isthe magnitude of a magnetic field that is required to switch, or changethe direction of, the magnetization of the sense layer or referencelayer. In other words, the sense or reference magnetization 210, 230 ofthe different MLU cells 10 can switch at different magnitudes of theread magnetic field 44. Each MLU cell 10 can further have individualelectrical variation characteristics.

The MLU device 100 can comprise a processing unit 51 configured forcomputing a electrical variation from the sense signal 31 (see FIGS. 5and 6).

The authentication method using the MLU device 100 can further comprisea step of using the processing unit 51 for computing the electricalvariation from the sense signal 31. The computed electrical variationcorresponds to an electrical variation signature 50 since it ischaracteristic of the MLU device 100 from which it has been obtained.Indeed, a unique electrical variation signature 50 is computed for a MLUdevice 100 that translates the process variability during themanufacturing of the MLU device 100, due to the distinct characteristicsrelated to the magnetic portions of the MLU cell layers from a MLU cellto another MLU cell 10.

The electrical variation can comprise a variation directly measured involt V by a voltmeter. Alternatively, the electrical variation cancomprise a variation expressed in V²/Hz, or V/sqrt(Hz) or VN/sqrt(Hz)that can be measured by using a spectral analyser. For example, theelectrical variation can comprise a noise spectral density.

In an embodiment, the processing unit 51 comprises a spectrum analyzerdestined to compute the noise spectral density signature 50. Thespectrum analyzer 51 calculates the magnitude of the sense signal 31versus frequency within the full frequency range of the spectrumanalyzer. In fact, any suitable device for measuring a voltage variationat a given frequency can be included in the processing unit 51 tocompute the noise spectral density signature 50.

The noise spectral density signature 50 can be computed at a singlefrequency for all or part of the magnitudes of the varying read magneticfield 44.

Alternatively, the noise spectral density signature 50 can be computedat any frequencies for all or part of the magnitudes of the varying readmagnetic field 44. For example, the noise spectral density signature 50can be computed at frequencies between 0.1 Hz and 10 kHz.

In another variant, the noise spectral density signature 50 can becomputed across a bandwidth for all or part of the magnitudes of thevarying input signal 43 (thus varying read magnetic field 44).

FIG. 4 reports the noise spectral density signature 50 as a function ofthe read magnetic field 44 for three different MLU devices 100 havingthe same configuration. Due to process variability during themanufacturing of the MLU devices 100, three distinct noise spectraldensity signatures are measured for the three MLU devices 100.

According to an embodiment, the processing unit further comprises acomparison circuit 53 and the authentication method further comprises astep of using the comparison circuit 53 to compare the computed noisespectral density signature 50 of the MLU device 100 under investigationto a reference noise spectral density signature 52. It can be understoodthat the reference noise spectral density signature 52 can be a noisespectral density that has been computed from the sense signal 31 of areference MLU device 100.

Comparing the computed noise spectral density signature 50 to thereference noise spectral density signature 52 can be performed by usingcharacteristics features of the noise spectral density signature such aspeaks, valleys, saddle points or steps. Comparing the computed noisespectral density signature 50 to the reference noise spectral densitysignature 52 can further be performed by using the spectrum shape. Forinstance, the noise could have a 1/f or a 1/f² dependence in givenconditions.

FIGS. 5 and 6 illustrate schematically an exemplary MLU device 100 andthe authentication method, according to an embodiment. The read magneticfield 44is provided to the MLU device 100 such as to orient the sensemagnetizations of the MLU cells 10 and vary the total resistance R ofthe MLU device 100. The sense signal 31 is provided in the bit line 3such that the total resistance R of the MLU device 100 can be measured.The processing unit 51 is used for computing a noise spectral densityfrom the sense signal 31 and outputting the noise spectral densitysignature 50. The comparison circuit 53 is then used to compare theoutputted noise spectral density signature 50 to the reference noisespectral density signature 52.

In FIG. 5, the noise spectral density signature 50 matches the referencenoise spectral density signature 52. This can be seen from thesuperposition 50+52 of the reference noise spectral density signature 52and the noise spectral density signature 50. In FIG. 6, the noisespectral density signature 50 does not match the reference noisespectral density signature 52.

The comparison circuit 53 can be further configured for outputting anoutput signal 54 comprising information on the match or non-matchbetween the noise spectral density signature 50 and the reference noisespectral density signature 52. For example, in the case the noisespectral density signature 50 matches the reference noise spectraldensity signature 52, the output signal 54 can correspond to a logical“1” data value. If the noise spectral density signature 50 does notmatch the reference noise spectral density signature 52, the outputsignal 54 can correspond to a logical “0” data value.

In an embodiment, the noise spectral density signature 50 measured forthe MLU device 100 may be configured for generating physical unclonablefunctions (PUFs). For example, a MLU device 100 may be identified usingPUFs. A PUF implements challenge-response authentication, where oneparty presents a question (“challenge”) and another party must provide aproper answer (“authentication response”) to be authenticated.

Here, the read magnetic field 44provided to the MLU device 100 underinvestigation (challenged MLU device 100) corresponds to the challenge.The comparison circuit 53, comparing the noise spectral densitysignature 50 of the challenged MLU device 100 to the reference noisespectral density signature 52 of the reference MLU device 100, may beconfigured to generate a PUF output bit (output signal) 54,corresponding to the authentication response.

The PUF output bit 54 may thus be used as a product identifier touniquely identify a challenged MLU device 100 based on manufacturingprocess variations that results in a particular (unique) noise spectraldensity signature, when the challenged MLU device 100 is compared to areference MLU device 100, when the challenged MLU device 100 and thereference MLU device 100 are written with the same programmed state.

In an embodiment, there is provided a computer program product embodiedin a non-transitory computer readable medium and comprising computercode for causing the processing unit 51 to execute the authenticationmethod items.

REFERENCE NUMBERS AND SYMBOLS

-   10 MLU-based magnetic cell-   100 MLU-based magnetic device-   21 first magnetic layer, sense layer-   210 first magnetization, sense magnetization-   22 spacer layer-   23 second magnetic layer, storage layer-   230 second magnetization, storage magnetization-   24 antiferromagnetic layer-   3 bit line-   3 a first branch-   3 b second branch-   31 sense current, sense signal-   32 heating current-   4 field line, input device-   41 write current-   42 write magnetic field-   43 read current-   44 read magnetic field, input signal-   50 noise spectral density signature-   51 noise measurement, spectrum analyzer-   52 reference noise spectral density signature-   53 comparison circuit-   54 output signal, PUF output bit-   R resistance of the ML device

1-16. (canceled)
 17. A magnetic logic unit (MLU)-based magnetic devicecomprising one or a plurality of MLU-based magnetic cells, each MLU cellcomprising a first ferromagnetic layer having a first magnetization, asecond ferromagnetic layer having a second magnetization, and a spacinglayer between the first and second ferromagnetic layers; an input deviceconfigured for generating an input signal adapted for changing theorientation of the first magnetization relative to the secondmagnetization, in the plurality of MLU cells, and vary a resistance ofthe MLU device; a bit line configured for passing a sense signal adaptedfor measuring said resistance; and a processing unit; wherein the inputsignal comprises a read magnetic field having a magnitude that varies byincrement steps; the processing unit being configured for computing anelectrical variation from the sense signal and outputting an electricalvariation signature; the outputted electrical variation signaturecorresponding to the sense of the different MLU cells switching atdifferent magnitudes of the read magnetic field, the processing unitcomprises a comparison circuit configured for comparing the electricalvariation signature to a reference electrical variation signaturecomputed for a reference MLU device and outputting an output signalcomprising information on the match or non-match between the electricalvariation signature and the reference electrical variation signature,wherein said electrical variation comprises a noise spectral density,and the processing unit comprises a noise spectrum analyzer configuredfor computing a noise spectral density signature.
 18. The MLU deviceaccording to claim 17, comprising a plurality of MLU-based magneticcells, wherein each of said plurality of MLU cells is electricallyconnected in series or in parallel via the bit line or in a combinationof series and parallel configurations via the bit line.
 19. The MLUdevice according to claim 17, wherein said MLU device comprises a bitline forming a first branch and connecting in series at least two MLUcells; and further comprises another bit line forming a second branchand connecting in series at least two MLU cells; and wherein the twobranches are electrically connected in parallel.
 20. The MLU deviceaccording to claim 17, wherein said output signal corresponds to alogical “1” data value in the case of a match and to a logical “0” datavalue in the case of non-match or wherein said output signal correspondsto a logical “0” data value in the case of a match and to a logical “1”data value in the case of non-match.
 21. The MLU device according toclaim 17, configured for generating physical unclonable functions (PUFs)wherein the read magnetic field is usable as challenge and the outputsignal is usable as response.
 22. The MLU device according to claim 17,wherein the input device includes a field line adapted for passing aread current generating the read magnetic field.
 23. An authenticationmethod using a MLU device comprising one or a plurality of MLU-basedmagnetic cells, each MLU cell comprising a first ferromagnetic layerhaving a first magnetization, a second ferromagnetic layer having asecond magnetization, and a spacing layer between the first and secondferromagnetic layers; an input device configured for generating an inputsignal adapted for changing the orientation of the first magnetizationrelative to the second magnetization, in the plurality of MLU cells, andvary a resistance of the MLU device; a bit line configured for passing asense signal adapted for measuring said resistance; and a processingunit; wherein the input signal comprises a read magnetic field having amagnitude that varies by increment steps; the processing unit beingconfigured for computing an electrical variation from the sense signaland outputting an electrical variation signature; the outputtedelectrical variation signature corresponding to the sense of thedifferent MLU cells switching at different magnitudes of the readmagnetic field; the outputted electrical variation signaturecorresponding to the sense of the different MLU cells switching atdifferent magnitudes of the read magnetic field; wherein the processingunit comprises a comparison circuit configured for comparing theelectrical variation signature to a reference electrical variationsignature computed for a reference MLU device and outputting an outputsignal comprising information on the match or non-match between theelectrical variation signature and the reference electrical variationsignature; and wherein said electrical variation comprises a noisespectral density, and the processing unit comprises a noise spectrumanalyzer configured for computing a noise spectral density signature;the method comprising: providing an input signal for changing theorientation of the first magnetization relative to the secondmagnetization and vary a resistance of the MLU device; providing a sensesignal in the bit line for measuring said resistance; wherein the inputsignal comprises a read magnetic field and the method comprises varyinga magnitude by increment steps of the read magnetic field; computing anelectrical variation from the sense signal and outputting an electricalvariation signature; the outputted electrical variation signaturecorresponding to the sense of the different MLU cells switching atdifferent magnitudes of the read magnetic field.
 24. The methodaccording to claim 23, wherein said electrical variation comprises anoise spectral density.
 25. The method according to claims 24, whereinthe noise spectral density is computed at a single frequency or at aplurality of frequencies for all or part of the magnitudes of thevarying read magnetic field.
 26. The method according to claim 25,wherein the noise spectral density is computed at frequencies between0.1 Hz and 10 kHz.
 27. The method according to claim 25, wherein thenoise spectral density is computed across a bandwidth for all or part ofthe magnitudes of the varying read magnetic field (44).
 28. The methodaccording to claim 23, wherein the processing unit further comprises acomparison circuit; and wherein the method comprises a step of using thecomparison circuit to compare the electrical variation signature to areference noise electrical variation signature; and outputting an outputsignal comprising information on the match or non-match between theelectrical variation signature and the reference electrical variationsignature.
 29. A computer program product embodied in a non-transitorycomputer readable medium and comprising computer code for causing theprocessing unit to execute the method of claim 23.